1. Field of the Invention
The present invention relates to an optical resonator type organic electroluminescent element (hereafter referred to as the organic EL element), which is provided with a luminescent layer having an organic material held between a pair of electrodes, makes the luminescent layer emit light by injecting a carrier from both electrodes into the luminescent layer, and releases electromagnetic radiation in the form of light by resonating a specific wavelength in the emitted light.
2. Description of the Related Art
Research is being carried out into a flat-panel display in which the organic EL element and a plane source of light are used, as the next generation display materials, and they have attracted considerable attention. A simple dot matrix display is now being developed so as to be put to practical use. The organic EL element has a luminescent layer of an organic material formed between an anode electrode and a cathode electrode, and electrons and electron holes, which are injected from the two electrodes into the luminescent layer, are recombined therein to generate light of a fluorescence spectrum corresponding to the organic material. This light is externally emitted as a natural emitted light without directivity from the organic EL element through a glass substrate.
In order to provide such an organic EL element which can emit light with directivity, there is proposed an organic EL element having a minute optical resonator structure as shown in FIG. 1. The organic EL element having this resonator structure has an electron hole transportation layer 14 and a luminescent layer 16 of the organic material between an anode transparent electrode 12 and a cathode metallic electrode 50, and it also has a multilayered film mirror 40 between a glass substrate 10 and the anode electrode 12. The minute optical resonance structure comprises the multilayered film mirror 40 and the metallic electrode 50, and the light of a specific resonance wavelength, which is defined by a space between the multilayered film mirror 40 and the metallic electrode 50, is amplified. When a luminescent material having a broad emission spectrum is used as the organic luminescent layer 16, however, it is difficult to actually impart directivity to the luminescence light.
It is reported that the directivity of light toward the front of the element can be improved by accurately controlling the space between the multilayered film mirror 40 and the metallic electrode 50, as well as the position of a resonance wavelength, and also determining the optical length to a length of 1.5 times as long as a target wavelength xcex (JPA (Hei) 9-180883).
However, the above-mentioned organic EL element having an optical length of 1.5xcex had an anode transparent electrode having a thickness of about 30 nm.
Even if the transparent electrode is ITO (indium tin oxide) having a high electric conductivity, its sheet resistance is as high as 30 xcexa9/xe2x96xa1, and it is impossible to prevent the generation of Joule heat. For example, when this organic EL element is used as an extremely bright backlight of a liquid crystal display device, even the most highly effective EL element requires a large inflowing current of about 100 mA/cm2 or more. The above-described element is normally driven at a current of 10 mA/cm2 or less as a display or the like. If a much larger current than that is caused to flow, heat generation in the ITO electrode cannot be avoided, and the organic EL element is heavily deteriorated as the temperature becomes higher because its deterioration is directly related to temperature. Experience shows that the lifetime of the element is decreased to {fraction (1/10)} as the driving current is increased by ten times.
For example, it is reported in Applied Physics Letter 65(15) xe2x80x9cSharply directed emission in organic electroluminescent diodes with an optical-microcavity structurexe2x80x9d, Oct. 10, 1994 that use of a special luminescent material (metal complex containing a rare earth element) realizes a high directivity of light toward the front of the element by the ITO anode electrode with a thickness of 158 nm and substantially the same configuration as the conventional one.
However, the special luminescent material described in the above-mentioned document has a low luminous efficiency and unstable chemical performance. Therefore, it does not provide a highly effective element with high brightness. In addition, the organic layer has a total film thickness of 100 nm or less, and the reliability of the element is low in view of a withstand voltage or the like due to a defect or the like, and the organic layer has high probability of a local short-circuit breakdown. Thus, the above-described element is not realistic for practical use.
As described above, an optimum optical resonant organic EL element has not been provided yet, and there is not provided a resonant organic EL element which uses a luminescent material having high luminous efficiency and has high reliability at a large current. Besides, the satisfactory directivity of light toward the front of the element has not been achieved.
The present invention was achieved to solve the above-mentioned problems, and it is an object of the invention to improve the luminous efficiency and reliability of an organic EL element.
To achieve the above-mentioned object, an organic electroluminescent element having a minute optical resonator for amplifying a specific wavelength in a luminescence light comprising a substrate; a multilayered film mirror formed of laminated layers on the substrate, each of the layers having a different refractive index; a transparent electrode as an anode on the multilayered film mirror; an organic layer on the transparent electrode; and a metallic electrode mirror as a cathode on the organic layer, said organic layer comprising a luminescent layer for emitting a light by injecting electron holes and electrons through the transparent electrode and the metallic electrode mirror, said multilayered film mirror and said metallic electrode mirror constituting a minute optical resonator for amplifying a specific wavelength in the light, wherein the minute optical resonator has an optical length twice as long as a target amplified wavelength, the organic layer is not less than 100 nm thick, and the transparent electrode is not less than 50 nm thick, or is of not more than 30 xcexa9 in sheet resistance.
The xe2x80x9coptical lengthxe2x80x9d of the minute optical resonator is determined by a soaked quantity of light into the multilayered film mirror and an optical thickness of the organic layer (Document: J. Appl. Phys., 80 (1996) 6954). The optical length is determined to be twice-as long as a target amplified wavelength xcex so to make it possible for the anode transparent electrode and the organic layer to have an appropriate thickness in view of the characteristics of the element.
Specifically, it is characterized by determining the organic layer to have a thickness of 100 nm or more, and the transparent electrode to have a thickness of 50 nm or more or a thickness so to have a sheet resistance of 30 xcexa9/xe2x96xa1 or less. The reliability of the organic layer is improved by determining its thickness to 100 nm or more. Heat generation due to a large current flowing through the element can be suppressed by setting the transparent electrode to have a thickness of 50 nm or more or a thickness so to have a sheet resistance of 30 xcexa9/xe2x96xa1 or less. Thus, the characteristics of the element can be prevented from being deteriorated due to heat generation, and the element can be driven at a large current and readily employed as a lighting device with high brightness.
Moreover, it is suitable in this invention to adjust the above-mentioned target amplified wavelength in a range of about 30 nm toward the side of a shorter wavelength from the luminescence peak wavelength of the luminescent layer.
Because a satisfactory emission intensity or high directivity cannot be obtained if the target amplified wavelength is more than 30 nm away from the luminescence peak wavelength of the luminescent layer, the conditions described above are preferably determined so that a satisfactory emission intensity can be obtained, and light having high directivity can be emitted from the element.
It is more suitable to set the transparent electrode to have an optical thickness of xcex/2 for example. Here, an optical thickness L of the film is expressed in L=Dxc3x97n (n: a refractive index) with respect to an actual thickness D of the film. By setting it as described above, the transparent electrode has an actual thickness of about 129 nm (actual thickness is a value resulting from dividing the optical thickness by its refractive index (=1.93)) when the target amplified wavelength xcex is for example 500 nm, and a sheet resistance of about 15 xcexa9/xe2x96xa1 can be achieved.
Moreover, it is desirable to use the luminescent material with a narrow emission spectrum to configure the organic layer in order to additionally enhance the luminescence color purity of the organic EL element. For example, it is desirable to use a luminescent material such as quinacridone having a half-width of about 80 nm or less.